Nickel,cobalt,chromium steel



March 24, 1970 D. s. DABKOWSKI ETAL 3,502,462

NICKEL, COBALT, CHROMIUM STEEL Filed Nov. 29, 1965 Upper Boundary-Conventional Steels Mm mm 0 I80 I90 YIELD STRENGTH (0.2 OFFSET). ks!

am RS E 0w mufi v T0 00 y NML B M E 0 H r m mn m m VWR /l/f SKW. A J L M m DMLMfi w United States Patent 07 U.S. Cl. 75-128 1 Claim ABSTRACT OF THE DISCLOSURE A steel of high yield strength and good notch toughness adapted for use in wrought form as well as weld filler metal has the essential composition, by weight: nickel 95-14%; cobalt 610%; carbon 0.060.l6%; molybdenum 0.7-l.5%; chromium .53%; and the balance iron except for residuals and minor impurities.

This invention relates to high yield strength steels having good notch toughness and to high yield strength weld metal of comparable properties.

In many applications where weight of the structure or thickness of the material must be maintained at a minimum, steels combining high yield strength and good notch toughness are required. Among the uses for which such steels are particularly suitable are the construction of pressure vessels, missile motor cases, submarine hulls, nuclear reactor pressure vessels and highly stressed structural members. Inasmuch as such uses require Welding operations, it is also essential that the whole weldment including the weld metal possess superior properties.

There is an urgent need for high-toughness filler metals for joining constructional alloy steels with yield strengths in the range 160 to 2.20 K s.i. Such filler metals, after deposition by conventional fusion welding processes, must have high toughness and yield strengths approximately equal to those of the base steel in either the as-welded condition or after a relatively simple postweld heat treatment.

Two types of filler metals have been used to join highyield-strength constructural alloys in the yield-strength range 160 to 220 K s.i. One group of filter metals, the maraging alloy steels, can be used to produce deposited and aged weld metal with satisfactory yield strengths but with only marginal toughness (30 to 50 ft.-lbs. Charpy V-notch energy absorption at F.). Moreover, the maraging filler metals are not particularly compatible with conventional carbon-containing alloy steels. The second group of filler metals, carbon-containing nickel alloy steels, are generally used in the as-welded condition and exhibit moderately good toughness (40 to 60 ft.-lbs. Charpy V-notch energy absorption at 0 F.) However, to achieve a yield strength of 160 K s.i. and higher, these nickel alloy steels must contain about 0.20 to 0.30 percent carbon and, as a result, such alloys exhibit a moderately high susceptibility to weld cracking.

In the past several years, considerable data have been obtained on the yield strength-notch toughness relationship of a variety of alloy steels. These data indicate that there is an upper limit to the notch toughness of such steels. Moreover, it has been found that the notchtoughness limit decreases as the yield strength increases, particularly in steels with strength levels of 160 K s.i. or greater. In these high strength steels, the upper limit of notch toughness is about 60 -ft.-lbs. (Charpy V-notch energy absorption at 0 F.) at 180 K s.i., and drops below '40 ft.-lbs. (Charpy V-notch energy absorption at 0 F.)

at 200 K s.i.

3,502,462 Patented Mar. 24, 1970 Studies compiled by the Naval Research Laboratory have established an upper boundary strength-to-toughness curve for conventional high yield strength steels. The results of the study have been published in NRL Report 6300, June 1965, available from the Clearinghouse for Federal Scientific and Technical Information (CFSTI), Sills Building, 5285 Port Royal Road, Springfield, Va.

The present invention involves high strength steel having notch toughness in excess of the above mentioned limits and generally having a yield strength-to-toughness relationship which is better than conventional high yield strength steels. Steels according to the invention combine the strengthening principles of quenched and tempered steels with the strengthening principles of maraging steels and thus obtain a part of the strengthening from carbide precipitation and a part from precipitation of intermetallic compounds. Such steels exhibit the advantages of both quenched and tempered and maraging steels without the accompany disadvantages of these types of steels. Moreover, weld metal of such compositions combine the resistance to weld cracking of low carbon maraging steels with the superior toughness of nickel-cobalt steels.

Quenched and tempered steels require a carbon content in excess of about 0.2% to exhibit yield strength in to 200 K s.i. range. At this carbon level, weldability is poor because the steel is subject to excessive cracking in the zone affected by weld heat and the maximum notch toughness obtainable is limited because of the brittleness of the carbides that strengthen these steels. However, the production of carbon-containing quenched and tempered steels is economical and such steels are commonly used for a variety of applications.

Maraging steels are essentially carbon free and obtain strength from the precipitation of complex nickel-molybdenum and nickel-titanium intermetallic compounds and obtain additional strength from undetermined mechanisms involving molybdenum and cobalt. Higher levels of yield strength and notch toughness can be developed in maraging steels because of the superiority of the strengthening mechanisms in these steels over the car- :bide precipitation strengthening mechanism in the quenched and tempered steels. However, because of the presence of titanium and aluminum in the maraging steels, care must be taken during melting to obtain the lowest possible levels of carbon, sulphur, nitrogen and oxygen. This is necessary to prevent the formation of particularly harmful particles of TiC, TiS, TiN, AlN, TiO A1 0 which adversely affect notch toughness. Thus, melting practices for maraging steels are relatively expensive and time consuming and special vacuum melting practices are required to develop optimum properties.

The present invention concerns a steel composition and weld metal for joining members thereof in which a part of the strengthening is obtained by the formation of nickel-molybdenum intermetallic compounds and part by the strengthening elfects of cobalt. In these steels, it is possible to reduce the carbon content to below about 0.16% and thereby obtain good notch toughness. At this carbon level, some strengthening will result from the formation of carbides but the carbon content will not be so high that the weldability and notch toughness will be adversely afiected. Such steels attain yield strength levels greater than 160 K s.i. but will not be as sensitive to the level of residual elements sulfur, nitrogen and oxygen, as are the maraging steels because these steels are substantially free of titanium and aluminum which are primarily responsible for the aforementioned harmful sulfides, carbides and nitrides. Weld metal of our steel compositions, quite unexpectedly, has a yield strength and toughness at least as good as the wrought forms.

3 According to the invention, there is provided a steel with high yield strength and good notch toughness which consists essentially of at least 9.5% nickel, preferably 9.5 to 14% nickel, at least 6% cobalt, preferably 6 to 10% cobalt, about 0.06 to 0.16% carbon, preferably 0.1 to

0.16% carbon, about 0.7 to 1.5% molybdenum, 0.5 to 3% chromium, preferably 1 to 2% chromium, and the balance substantially iron. The expression substantially iron means that the steels may contain small amounts,

e.g. less than about 0.7% total of other elements such as in Table I.

TABLE I S Si Ni Cr Mo C0 A1 1 N O 1 Acid soluble.

manganese, silicon and aluminum, (the latter up to about 0.025% max.) to perform their common functions and residual amounts of sulfur and phosphorous. The preferred composition is a steel which consists essentially of 9.5 to 12% nickel, 6 to 10% cobalt, 0.1 to 0.16% carbon, 0.7 to 1.5% molybdenum, 1 to 2% chromium and the balance substantially iron. It has been found that Samples of these compositions were strengthened by austenitizing, water quenching and aging in a conventional manner and then subjected to mechanical property evaluation. The results of this evaluation and a comparison of these results with the properties of as-quenched samples are presented in Table II.

TABLE 11 Yield Oharpy V-notch strength energy absorption,

(0.2 U Tensile Elongation Reduction Hardit.-1b. Aging ofiset), strength, in 1 inch, of area, ness, temp., F. K s.i. K s.i. percent percent R +80 F. 0 F.

Steel.

A As-quenehed 166 211 17. 0 66.3 45. 5 56 48 400 164 207 16. 0 65. 9 46.0 50 53 600 167 192 16. 0 68. 3 44. 0 52 50 800 168 181 16. 0 66 8 42. 0 54 42 900 156 162 19. 0 70. 2 39.0 84 65 1, 000 131 140 22. 0 74. 9 34. 0 124 124 1,100 112 130 24. 0 76 7 30. 0 157 141 1, 200 99 138 24. 0 73 0 31. 5 131 130 B As-quenehed 170 228 6. 0 10.4 48. 5 4 4 400 180 226 5. 0 9. 0 48. 5 4 4 600 186 211 5. 5 13. 8 46. 5 4 4 800 187 206 5. 0 16. 2 45. 5 4 4 900 191 205 5.0 9. 7 50. 0 1, 000 202 209 4. 0 10. 5 47. 0 4 3 1, 100 154 156 2. 0 5. 6 39. 0 3 2 G As-quenched 172 233 8. 0 22.8 49. 5 6 5 400 189 223 11. 0 39. 0 48. 0 5 5 600 178 189 12. 0 50. 5 44. 5 7 3 800 145 149 16.0 55. 7 36. 0 12 9 900 128 133 19. 5 60. 1 31. 0 20 15 1,000 117 121 23. 0 65. 1 27. 0 34 26 1,100 103 113 28. 5 66. 4 25.0 53 1,200 87 145 19. 0 18. 5 31. 5 13 11 D As-quenehed 158 207 17. 0 65. 3 44. 5 47 400 164 201 17. 0 66. 4 45. 0 51 51 600 171 199 16. 0 62. 2 44. 5 46 44 800' 172 199 18. 0 65. 2 44. 5 42 39 900 183 203 17. 0 67. 7 46. 0 50 49 1, 000 180 191 16. 5 70. 8 44. 5 56 1, 137 151 22. 0 75.4 35. 0 121 111 1, 200 114 155 21. 0 72. 8 35. 0 112 E As-quenched 158 208 16. 0 65. 9 45. 5 56 58 400 155 198 16. 0 67.9 45. 0 64 69 600 159 193 16. 0 67. 8 49. 0 60 61 800 171 204 17. 0 66. 5 46. 0 51 46 900 184 207 16. 0 67. 7 47. 0 58 60 1,000 172 181 18.0 71. 7 41. 5 87 88 1,100 1 4 149 22. 0 75. 2 35. 5 122 1, 200 105 148 21. 0 74. 4 33. 5 129 F, As-quenched 177 228 14. 0 55. 2 48. 0 22 22 400 194 227 15. 0 58. 6 47. 0 24 24 600 195 222 14. 5 56. 8 47. 5 21 20 800 206 229 14. 0 56. 4 48. 0 21 20 900 207 223 16. 0 59. 6 48. 5 21 19 1, 000 186 190 16. 0 61. 0 43. 0 27 24 1, 100 132 152 21. 0 69. 1 35. 0 54 48 l: 200 114 49 20. 0 68. 8 3 0 71 66 TABLE. II-Comtin ued Yield Oharpy V-notch strength energy absorption, (0.2% Tensile Elongation Reduction Hardft.-lb. Aging oflset) strength, in 1 inch, of area, ness, tempz, F. K s.i. K s.i. percent percent R +80'F. F.

G Asquenched 184 236 15. 0 F4. 3 50. 0 38 41 400 176 224 16. 0 65. 6 47. 0 43 40 600 177 214 16. 0 65. 0 47. 0 35 35 800 184 223 17. 0 65. 3 48. 0 32 33 900 203 227 16.0 66. 9 49. 0 40 37 1, 000 185 191 17. 71. 1 45.0 63 62 1, 100 142 161 22. 0 74. 6 38. 0 104 105 1, 200 120 158 21. 0 73. 0 36. 0 109 104 H As-quenched 199 263 14. 5 56. 4 51. 0 20 18 190 242 14. 0 58. 7 50. 0 24 24 I As-quenehed 165 212 16. 0 65. 6 46. 5 50 49 400 172 206 15. 0 67. 4 46. 0 54 52 600 173 201 16. 0 66. 5 45. 0 47 43 800 177 200 16. 0 65. 7 46. 0 46 44 900 181 199 16. O 68. 2 46. 0 58 53 1, 000 177 189 18. 0 72. 3 44. 5 68 70 1, 100 135 149 22. 0 77. 1 36. 0 121 114 1, 200 112 155 22. 0 71. 6 36. 0 109 106 .T As-quenched 153 200 16. 0 67. 2 44. 0 65 68 400 151 190 17. 0 67. 9 43. 0 74 75 600 153 186 17. 0 67. 5 42. 0 69 68 800 I61 191 18. 0 68. 2 44. 0 65 61 900 171 194 18. 0 68. 7 44. 0 74 73 1, 000 163 173 20. 0 72. 2 40. 0 96 98 1, 100 126 145 22. 0 77. 0 33. 0 142 143 1,200 108 145 21. 0 74. 6 34. 0 139 147 %-inch-thick plate samples austenitized at 1,500 F. for 1 hour and water-quenched, then aged for 5 hours at the indicated temperature, and water-quenched.

The results reported in Table II show that both chromium and molybdenum are necessary in the high nickelcobalt steels to obtain good notch toughness (samples A, B, C and D) and that changes in chromium content within the 1 to 2% range have little eifect on the yield strength and notch toughness of these steels except that at equivalent strengths, the 2% chromium-1% molybdenum steel exhibited about a ft.-lb. higher Charpy V-notch energy absorption at 0 F. (samples D and E).

It is also noted that the yield strength at the peak aging temperature of 900 F. increased from 184 to 203 K s.i. when the carbon was increased from 0.12 to 0.2% and the notch toughness decreased progressively from about 60 ft.-lbs. (Charpy V-n'otch) for the 0.12% carbon sample to about ft.-lbs. in the 0.2% carbon samples (samples E, G and H). Changes in the cobalt content from 6 to 8% have little effect on the strength and toughness of the high nickel-chromium-molybdenum steel (samples D and I), but steels with 8% cobalt are superior to steels with 6% cobalt and 2% chromium (samples E and J).

One unusual feature of our steels is the manner in which the notch toughness starts to increase at aging temperatures below those at which the yield strength reaches its peak. This is contrary to the behavior of conventional quenched and tempered steels and is indirect evidence that the strengthening mechanism described in our steels is different from that involved in conventional quenched and tempered steels. The increasing toughness at aging temperatures of 900 to 1000 F. makes it possible to obtain still better combinations of strength and toughness.

It is also apparent from the data in Table II that a strong aging peak occurs at an aging temperature of 900 F. This is the optimum aging temperature for maraging steels and thus provides indirect evidence that the strengthening mechanisms operating in our steels are similar to those operating in maraging steels as Well. It is also observed that at the peak aging temperature, the increase in strength realized from an increase in carbon content of from 0.16 to 0.20% is less than the increase in strength realized from an increase in carbon of from 0.12 to 0.16%. This indicates that the optimum carbon content is no higher than 0.16% C.

It is also apparent from Table II that steels according to the invention are capable of achieving a notch toughness of about ft.-l=bs. (C'harpy V-notch energy at 0 F.) at a yield strength of K s.i. and over 60 ft.-lbs. at a yield strength of over 180 K s.i. These levels of notch toughness are substantially higher than those obtained in quenched and tempered steels strengthened primarily by carbide precipitation.

Of considerable importance, however, is the discovery that it is necessary to maintain controlled nickel and cobalt contents to providea steel with yield strength-to-toughness relationships better than convenional high strength steels. As discussed above, studies of previous investigators compiled by the Naval Research Laboratory have established an upper boundary curve for conventional high yield strength steels. This curve is reproduced in. FIGURE 1 and shows the aforementioned upper boundary limit of the strength and toughness of conventional steels in the range of to 200 K s.i. Also shown in FIGURE 1 are the yield strength and notch toughness of each of five steel compositions which differed only in nickel and cobalt contents. Compositions of these samples are shown in Table 111.

TABLE III Steel Ni Co C Mn P S Si CI M0 A L 12. 0 8. 0 12 0. 02 O. 01 0. 005 0. 10 2. 00 1. 00 003 M 10. 0 8. 0 12 0. 02 0. 01 0. 005 0. 10 2. 00 1. 00 003 N 9. 0 8. 0 l2 0. 02 0. 01 0. 005 0. 10 2 00 1. 0O 003 P 10. O 10. 0 12 0. 02 0. 01 0. 005 0. 10 2. 00 1. 00 003 R l0. 0 5. 0 l2 0. 02 0. 01 0. 005 0. l0 2. 00 i 1. 00 003 As can be seen from FIGURE 1, steels containing 9% nickel with 8% cobalt (sample N) and steels containing 10% nickel but only 5% cobalt (sample R) possessed a strength-to-toughness relationship below the upper boundary curve for conventional high yield strength steels. In contrasts, the steels in which the nickel was maintained above 9.5% and the cobalt at 6% or greater have yield strength and toughness exceeding the upper boundary curve (samples L, M and P).

At least 9.5% nickel is believed to be necessary to provide sufficient nickel so that strengthening through precipitation of nickel-molybdenum compound (Ni Mo) will occur. It has also been found that chromium contents n amounts of from 1 to 2% are desirable in these alloys to btain high notch toughness at high yield strengths and that molybdenum contents of 2.0% are in excess of the optinum level for good notch toughness in steels of this type.

As is known, the properties of metals can be generally bare wires and inert-gas-shielded tungsten-arc welding.

Weld filler metal of our composition may be used in the as-deposited condition if a yield strength on the low side of the 160 to 220 K s.i. range is desired. When maximum strength is required, a simple post weld aging mproved by metallurgical practices which include hot and treatment may be used. Aging is not critical and normal old working and various heat treatments. Thus, wrought aging practices for the base metal can be used. orms of steel typically have the best properties. It is, When weldments are to be made on plates or sheets herefore, quite surprising to discover that as a weld-filler of similar compositions, a filler wire of the same composinetal the steel compositions within the purview of the in- 10 tion can also be used. If, however, a different steel base ention possess mechanical properties as good as or better is to be. welded then the composition of the filler wire han wrought samples. Welding involves coalescing metal should be selected so that, after mixing with the base hat is melted by local application of heat whereby a base and taking reactions With the atmosphere into account, netal and filler metal are fused together. We have found a weld metal with the required composition is produced. hat weld metal of the steel compositions according to the It is desirable to maintain the carbon content of the nvention, containing at least 9.5% nickel, preferably 9.5 weld metal at between about 0.10 to 0.16% to develop .0 12% nickel, at least 6% cobalt, preferably 6 to 10% the best strength levels with minimum loss in crack re- :obalt, 0.5 to 3% chromium, 0.7 to 1.5% molybdenum, sistance and toughness. Manganese, silicon, and aluminum ).06 to 0.16% carbon and the balance substantially iron, should be maintained at the lowest levels consistent with :an be made which possesses yield strengths greater than the g d deOXidatiOll 0 the molten W ld p001 us [60 K s.i. and notch toughness of 70 ft.-lbs. or greater increases in these elements cause a loss in toughness. Both ICharpy V-notch energy absorption at 0 F.). The Weld phosphorous and sulfur should be maintained at low netal may also contain small amounts, i.e. up to about levels because both elements reduce toughness and in- ).7% total, of other elements such as manganese, silicon crease susceptibility to hot shortness. The nickel content and aluminum (the latter up to about 0.025%) to perform should be at least 9.5%, preferably about 10% because :heir common functions, and residual amounts of phosa decrease causes a loss in hardenability and also a less phorous and sulfur. When used as a well-filler metal, a prepotent hardening reaction to occur upon postweld aging. Eerred composition consists essentially of 9.5 to about 10% An increase in nickel over about 10% increases the nickel, 6 to about 8% cobalt, 0.1 to 0.16% carbon, about tendency to form retained austenite which, in turn, del to 2% chromium, .7 to 1.25% molybdenum and the creases both strength and toughness; however, up to about aalance substantially iron. 12% nickel is satisfactory. The cobalt content should be The following examples typical of the improved weldat least 6%, preferably about 6 to 8% because cobalt tiller metal according to the invention. Samples of weld causes a strength increase and also increases the temperametal of each of four compositions shown i T bl IV ture of martensite formation which, in turn, increases the were prepared. resistance to Weld cracking. Large amounts of cobalt, TABLE IV however, e.g. over about 10%, cause an undesirable loss in toughness and hardenability. 0 P S S1 co or A1 Although the effects are not well understood, chrow 0.10 0.010 0.002 0. 000 0. 09 10.1 8.25 1.02 1.01 0. 002 mium contents in the range 1.0 to 2.0% appear to be bene- $33188? 8888 8:881 8:888 81 1 1838 8:81 518 8:88 888% ficial in that the Strength is increased with no apparent z 0.13 0.004 0.001 0. 000 0. 00 10.0 6.11 1.02 1.04 0. 001 loss in toughness. The optimum molybdenum content is about 0.7 to 1.25%, preferably 1.0%; at levels lower than Weld metal or compositions W and Y were tested in both 1.0%, an insufficient age-hardening reaction may occur, the unaged and aged Condition and Weld metal of COIH- and at levels significantly higher than 1.5%, a severe positions X and Z were tested after aging at 900 F. for l i toughness ill 09cm; 5 hours. The mechanical properties are reported in Table In view of the foregoing, it is apparent that various V and, for comparison, the mechanical properties of plate changes and modifications may be made within the pursamples of these same compositions are also included. view of the invention and, accordingly, the scope of the TABLE V Yield OharpyV-notch strength energy (0.2% Tensile Elongation Reduction absorption, ft.-lb. ofiset), strength, inlinch, of area Postweld heat treatment K s.i. K s.i. percent percent F. 0 F Steel; N d l R8880fiiif iulamaaamxrrazana11t5.:::::::::::::::::" E8 588 i818 65. 8 88"""19 W "888115481888;assassin;8585811863115sge'anb 'riz n siiii 18%. 388 i818 6788 28 ".18 X 188115881288;assassin;21501018011110 amers 10;.11: if; 1818 6688 8? 88 T Y 0888.1p 8 $8881smaina;05.185811satjjjjj15:13:33.: 1 5 8 388 1 858 65. 9 28 88 Y "i n i gi i jhiiiiiiiidfl iifihi5055601560 i i EH51: iii 383 i818 6788 83 88 Z 888115181828;5.1510055803001500;assistant first: ii? iii i818 6582 58 28 It is apparent from Table V above that the properties of 7Q invention should be limited only by the appended claims.

the Weld metal, unexpectedly, compare very favorably with those of rolled plate specimens. Conventional fusion welding can satisfactorily produce weldments and weld metal with superior properties, The data reported in Table V We claim:

1. A steel having high yield strength and good notch toughness consisting essentially of 9.5% to 14% nickel, 6% to 10% cobalt, 0.06 to 0.16% carbon, 0.7 to 1.5%

were obtained from Weld metal deposited using solid 75 molybdenum, 0.5 to 3% chromium and the balance sub- 9 10 stantially iron except for residuals and incidental im- 3,152,934 10/1964 Lula 75128.9 purities. 3,285,738 11/1966 Johnson 75128 References Cited 3,366,471 1/ 1968 Hill.

UNITED STATES PATENTS 5 HYLAND BIZOT, Primary Examiner 1,473,208 11/1923 Clement.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,502 ,462 March 24, 1970 Donald S. Dabkowski et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as 4 shown below:

Column 2, line 20, "accompany" should read accompanying Columns 3 and 4, TABLE II, Steel B, last line under heading "0 F", insert 5 Steel E, line 7, under heading "Yield Strength "l 4" should read 134 Steel G, line 1 of "Reduction of f area, percent" should read 64.3 Column 6, line 43, "convenional" should read conventional Columns 5 and 6, TABLE III, last column in the heading, "A should read Al Column 7, line 27, "well" should read weld Signed and sealed this 22nd day of December 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

